The reasonings leading to the theories which have attempted to
explain the cause of sporadic-E are as plausible as they are fallible. I intend here to
partially resurrect an old theory with a few embellishments, which I am aware is itself as
fallible as it is plausible. However, I shall introduce some recent scientific evidence in
an effort to substantiate my arguments.

Firstly, I would like to back-track to some of the theories that have
appeared in the past. In doing so I do not intend to deride any previous ideas, but to add
to them as there may not be one mechanism peculiar to the generation of sporadic-E, but
several in combination.

Theories

The weather-related theories contain some credible arguments but
unfortunately they are not all uniquely seasonally coincidental with sporadic-E. The
occurrence of jet streams is not necessarily a mid-year phenomenon; neither are
atmospheric gravity waves nor wind-shear. This is not to say that these elements are to be
dismissed, as each may generate a potential propagation medium or be a trigger in its own
right [1]. There is a possibility that jet-streams could cause some primary ionisation
which is later acted upon by some other element that increases its potential for carrying
signals at, say, 50 MHz. If this were so, then we might expect some propagation at lower
frequencies such as 24 or 28 MHz whenever this occurred. The same argument would apply to
atmospheric gravity waves and wind shear. Of course in this case the ionisation produced
could support propagation at any time of the year on these lower frequencies. The fact
that they preferentially do so in summer requires something else to be added to the
equation.

The one factor that is obvious is that the Sun is always in prevalence
in the hemisphere where sporadic-E predominates, so I must assume that this is relevant.
Peaks of solar activity have no bearing on the added intensity of sporadic-E; in fact the
opposite has been observed to be the case. In years of minimum solar activity, occurrence
and intensity are usually, but not always greater. However, one factor that follows the
sun about the equator is the tropical thunderstorm belt. These storms contain an enormous
amount of energy; each when released, is the equivalent of fifty atomic bombs. At any one
time there are eighteen hundred storms in progress around the world and one hundred
lightning strikes each second. The storms are concentrated mainly over land areas in the
equatorial regions which limits their occurrence to three blocks, namely Africa, South
America and South-East Asia. Only about ten per cent of the tropical storms occur over the
sea. So, as you can see, we have an enormous amount of potential energy to play with and
to produce elements which I offer as major contributors in the production of sporadic-E.

I have for some time favoured the lightning theory and more so in the
last couple of years due to effects observed during thunderstorms. I have experienced the
occurrence of sporadic-E shortly after thunderstorms which were detectable on six metres
in a south-easterly direction to be followed after several hours by sporadic-E towards the
east. Also some local storms together with a spectacular electrical storm, produced
sporadic-E afterwards. This was in Malta, where I was told that they often expect to have
E propagation after such thunderstorms. Now before you all get agitated and dash to your
radios after a thunderstorm, not every storm will produce sporadic-E. This will be
clarified shortly. There may be a combination of events necessary for its production, and
the storm may merely act as a trigger mechanism. I cannot state that thunderstorms are the
sole generator of sporadic-E, but read on.

Sprites

An article which appeared in Science in 1994 [2] aroused my curiosity
as it set my mind back to the age-old theory that thunderstorms could cause sporadic-E.
Some of you may recall the television broadcasts about upward-going lightning strikes, or
sprites as they were called, which have appeared since that publication. You may think
that these strikes do not go high enough to encounter the E-layer. This is partially
correct, as in my view, it is not the lightning itself that causes the ionisation.

The Compton Gamma-ray Observatory, carrying the Burst and Transient
Source Experiment (BATSE), was launched in 1991 to observe celestial gamma-ray sources. It
has detected numerous cosmic gamma-ray bursts and X-ray sources both persistent and
pulsed, together with several thousand solar flares. This is not a surprising revelation
as we would expect to gather this information as the norm. The observatory being designed
to detect events arriving at the earth from space, the sensors are orientated such that
they are able not only to detect the sources of such events, but to pinpoint their origin
in space. The observatory was not designed to detect earth-borne events and neither its
orbit path nor orbital period are favourable to covering large areas of the Earths
surface. Nevertheless, the observers were surprised to detect gamma-rays emanating from
the Earths atmosphere. By liaising with meteorological agencies they were able in
some cases to relate the emanations to thunderstorms. We are fortunate in this country
that being obsessed with the weather, we can obtain detailed forecasts and archive
information on our weather. A number of other countries also have fairly comprehensive
coverage of their weather but, unfortunately, there exist large areas of the globe that do
not enjoy this coverage. This is partly due to the fact that some countries do not need
this information in such detail as their weather systems are seasonal and to a large
extent predictable. The spacecrafts orbit takes it over such areas so precluding
correlation of gamma-ray events and thunderstorms in every case.

The critical part of the detection system mentioned above was that as
well as the detectors not being in an optimal positions to see the gamma
emissions, their trigger level threshold and gating time were not designed to detect these
weaker events. Fortunately another experiment aboard the spacecraft, the Orientated
Scintillation Spectrometer Experiment (OSSE) was able to confirm them.

The nature of a downward lightning strike should be familiar to most
people. As it strikes the ground it tends to disperse outwards like the roots of a tree
just below the surface, preferring the paths of high conductivity. This also produces an
ionised layer a metre or so above the surface where the dispersal is taking place.
Equally, we can imagine an upward-going lightning strike fanning out as it disperses into
the first conducting medium that it encounters. As there is no ground up there to provide
a solid conducting medium, it will tend to splay out over a large area and extend upwards
to a great altitude. As indicated above, the intensity of a lightning discharge is
immense. The enormous energy released by a single discharge will, like any other
discharge, produce radiation components. These will include the gamma-rays mentioned
already, as well as X-rays which are of the hard variety. By hard we mean they
will be in the short-wave part of the spectrum and hence very energetic. Also generated
will be Extreme Ultra-Violet radiation (EUV).

To avoid atmospheric absorption, the flashes of gamma radiation have to
occur at altitudes above 30km. Discharges observed from aircraft and space shuttles have
been seen to emanate from altitudes of 40 to 80km, each discharge having a vertical
component extending 10km upwards and 10 to 50 km horizontally. The intense field could not
only cause runaway electrons, but bremsstrahlung X-rays [5] to be generated. To accelerate
the electrons sufficiently the field would have to exceed 500V per metre over a path of
several kilometres to produce the megaelectron-volt electrons, and hence, the gamma-ray
bursts. The glow discharges observed from planes and space-shuttles could be sufficient to
generate those field intensities. This, as some of you will by now have surmised, brings
us dangerously close to the E-layer!

Ionisation mechanisms

The ionisation which occurs as we approach sunspot maximum has several
causes:

Solar radiation in the Extreme Ultra-violet (EUV) and X-ray
wavelengths

Galactic Cosmic rays (GCR)

energetic particles from the Sun and the Earths radiation
belts.

in the lower atmosphere, radioactive radiation from rocks.

The EUV and X-rays provide the most important source of
photo-ionisation above 60km. Radiation of all wavelengths reaches the F2-layer above
200kms, then the F1 layer at 140-200kms. The deep penetrating part of the EUV and X-rays
then encounter the E-layer at 90-140kms and finally, the D-layer at 70-90kms. During the
hours of darkness this still continues because of resonant scattering from the illuminated
side of the Earth, stellar continuum radiation (affecting the E-region) and galactic
cosmic rays (affecting the D-layer).

The chemical processes involved in ionisation are quite complex and I
will endeavour to give a fairly simple explanation of some of them. Some of the
constituents in the ionosphere are molecular ions of nitric and nitrous oxides and some
are atomic ions of oxygen, nitrogen, hydrogen and helium together with water. Now these
are the main components, but others less important to the present discussion are present
also. Because of gravitational separation the lighter components predominate with
increasing altitude. This means that as we rise up through the ionosphere the heavier
molecular ions give way to atomic ions with finally, in the F-layer, ions of oxygen,
helium then hydrogen and free electrons at the uppermost reaches. Now, those molecules
that are present in the upper reaches are subjected to the more intense radiation, and
such is the photon energy that dissociation from the molecular state to ionisation of the
atoms can occur all in one step. Keep this product in mind as you read later about the
gamma-ray and X-ray radiation.

In the F-layer the ion chemistry is primarily the conversion of O+ as a
primary ion into secondary molecular ions that recombine with electrons. In the E-layer
the primary products are molecular ions which are rapidly consumed in secondary reactions.
In this region we also find metallic ions which are reckoned to be the remnants of meteor
showers and seem to arrange themselves in narrow layers of one to three kilometres
thickness. These are what are assumed to be the carrier for sporadic-E. Metal atoms have
low ionisation potentials which means that they can be ionised by exothermic charge
transfer reactions alone and also being monatomic, are not easily neutralised. This also
implies that subjecting them to only moderate amounts of radiation will cause ionisation,
hence the possibility of ionisation by ionospheric wind-shear or jet streams.

Because of the greater density of the atmosphere, at the lower levels
the molecules are physically closer and recombination after ionisation occurs quickly.
This is the reason why I stated earlier that the gamma radiation has to start above 30km.
At higher altitudes the density is much less so that the mean free path lengths between
molecules and free electrons is much greater so that recombination takes much longer. The
ionised state therefore lasts much longer and replacement ions appear as fast as, if not
faster than they recombine, so giving us hours of HF propagation.

Sporadic-E

If we now look at this mechanism from an earthbound point of view with
the ionising mechanisms starting in the atmosphere and the gamma-rays moving upwards
through the D-layer and into the E-layer we see a possible generator for sporadic-E. If we
have intense, relatively localised ionisation fields being cumulatively produced by
successive strikes is it not possible that eventually E-clouds could be formed? There may
be a case for assuming that some of these clouds may form in the tropical and sub-tropical
areas and drift towards the poles explaining the more pronounced occurrence of E in
southerly latitudes. Then again, subjecting a partially ionised layer to a further
thunderstorm at a more northerly latitude could be sufficient to trigger propagation.

Now I come to the point where I tentatively offer an explanation of
trans-Atlantic propagation on six metres. What if the trans-Atlantic QSOs made on the
supposedly multi-hop E mode should turn out to be partially F-layer after all? This may
not seem such a wild statement if you will bear with me a while longer.

As I have explained before, normal F-layer propagation is the result of
EUV and X-rays. These are hard, which I defined as the short intense
penetrative rays that reach down into the D-layer, and the gamma-rays produced by
lightning fall into this same category. If the more intense of these rays travel up
through the E-layer they will encounter the F-layer with the distinct possibility of
localised ionisation. Whether or not this would be sufficient to cause the almost
loss-less propagation we often experience is conjecture. Just think of the
tiny pin-prick of mans interference with our upper atmosphere; rocket launches are
known to have burnt holes in the ionosphere due to the gases of hydrogen, carbon dioxide
and water vapour ejected, causing a halving of the electron concentrations through oxygen
ions recombining in great quantities (ATLAS-F 1982) [3]. So what price a massive
electrical storm now?

There is also a case for a combined effect as the E-layer would also be
subject to ionisation and produce a method of propagation akin to E-assisted TEP or
ducting across the Atlantic, and incidentally to the east as well if we are to consider
all eventualities. The signal, not crossing the equator or region of minimum dip, would
not be subjected to the flutter-fading that can happen on that path. The TEP
flutter-fading is said to be caused by the rise of a plasma cloud in the equatorial region
around sunset thus rapidly altering path-lengths. It is this plasma cloud that supports
the TEP propagation.

I have personally not noted any such effect on the trans-Atlantic QSOs,
even those which were made late at night and the early hours of the morning, when one
would expect this effect as the Sun would be setting somewhere along the path. The
propagation which I experienced in October 1987 involving E-assisted TEP to Botswana had
some flutter which the southerly stations did not note to any extent and may have been due
to the fact that I was located at the northerly extremity of the path.

If we had available ionosonde data during the openings we might see
some F-layer reflections but the blanketing effect of the E-layer could preclude detection
[4]. We can only measure the conditions at each end of the path as there is naturally a
dearth of land between here and North America.

The seasonal aspect of sporadic-E is undoubtedly solar related and the
fact that the tropical storm area cycles about the equator adds fuel to the argument. The
occasional burst of sporadic-E in December still has to be explained, although a
particularly intense event in the southern tropics could result in a northerly drift of a
section. It may be coincidental that this occurs about the time in their season as we
would expect a maximum in our northern summer season.

Conclusion

Studies, as previously mentioned, have been mainly concentrated in the
areas where there is a substantial amateur population and almost twenty-four hour
monitoring takes place. For us ever to understand fully the causes of sporadic-E would
entail a more detailed coverage of the areas involved. When you take into consideration
the area south of the Mediterranean reaching into the north and centre of Africa the
number of observers per unit area must be practically nil. The southern African states are
sparsely populated by amateurs and, even then, the proportion interested or active on VHF
who are able to monitor for more than a few hours a day must be miniscule compared with
the large area. The same conditions pertain with the large Japanese concentration to the
sparse Australian continent and the North America to South America circuit.

It is questionable whether with the present systems we will ever be
able to answer all the questions as to the cause of sporadic-E. One thing that is certain
is that you can never be sure that you have found the answer to a problem - you will
invariably find an answer, that is, until someone else turns up another answer.

Bremsstrahlung: this is X-radiation caused when an electrically charged particle is
slowed down by the electric field of an atomic nucleus.

I would like to express my thanks to my colleagues, Dr Isobel C Walker
and Dr Roderick Ferguson, at Heriot-Watt University for their help in producing this
article.

Addendum

After submission of this article another source of research in this
area has come to my attention in Scientific American, August 1997. In this
article by Messrs Mende, Sentman and Wescott they confirm that the discharge events are
taking place an altitude of about 90 km on a regular basis. The large potential
differences that exist between storm clouds and ground also exist between those same
clouds and the ionosphere which infers that the lightning can travel equally well in
either direction.

There is much in the way of research to be done in this area and the
technical challenges of investigating such short-lived events at such altitudes presents
the investigators with enormous problems which will, undoubtedly, take many years to
overcome.